[$] Shared memory mappings for devices

In a short filesystem-only discussion at the 2018 Linux Storage,Filesystem, and Memory-Management Summit (LSFMM), Ji(C)rime Glisse wanted totalk about some (more) changes to support GPUs, FPGAs, and RDMA devices.In other talks at LSFMM, he discussedchanges to struct page in support of these kinds of devices, but here he was looking to discussother changes to support mapping a device's memory into multiple processes. It should benoted that I had a hard time following the discussion in this session, sothere may be significant gaps in the article.